THE CURIOUS CASE OF SIDD FINCH By George Plimpton Macmillan. 275 pp. $14.95

TO AMATEUR sportsmen George Plimpton is the Ultimate Hero. He has dared to pitch to big league batters, to play quarterback in a pro football scrimmage and to guard the net as a pro hockey goalie. Such derring-do is the stuff of dreams for sports fans who, sensibily sitting in the stands, merely fantasize doing what George has actually done.

Plimpton's reasons for risking embarrassment and/or bodily harm on the diamond, the gridiron or the ice-rink did not stem from a simple-minded desire to prove his machismo. George wanted to write books about the people who play games for a living, and since first-hand experience is a writer's wellspring, Plimpton willingly went into the arena with the Lions of Detroit and the Bruins of Boston, among others.

To give him his due, George Plimpton is the best writer ever to participate in professional sports. His prose delineates the pro athlete's exceptional physical skills which he contrasts with his own average, adequate ability at sports. He captures the personality of the individual pro, thereby humanizing even the most arrogant of superstars. His impressions are invariabily perceptive and insightful. Above all, his sense of humor about sports, its players and himself make his books a pleasure to read.

Given all that, it is a puzzlement to find Plimpton's name on a novel -- his first novel at that -- about baseball. The Curious Case of Sidd Finch is enjoyable; George couldn't write badly if he tried. But a faithful reader of Plimpton's nonfiction sports books should rightly expect more than he gets this time around.

The genesis of the novel was a practical joke. Plimpton wrote an article for Sports Illustrated describing a phenomenal pitching prospect of the New York Mets who could throw a baseball 150 mph, half again faster than anyone has ever pitched. Sidd Finch, wrote Plimpton, was an aspiring Buddhist monk who played the French horn and was reluctant to commit himself to professional baseball. The article appeared on April Fools' Day and was so cleverly crafted that many readers believed it. One man, who took his sports seriously, was so offended when the joke was revealed that he cancelled his subscription to the magazine and to several other Time, Inc. publications.

The article was sketchy about Sidd Finch, the person. It ended with a number of hypotheses, the most important being the effect an unhittable pitcher would have on professional baseball. Would the commissioner, Peter Ueberroth, have to ban Sidd Finch for being not in the best interests of the game? It was all a delicious fraud, a coup for Plimpton and the magazine.

The Curious Case of Sidd Finch is the long, sometimes convoluted story about that short, funny article. It's sort of like a shaggy-dog tale that once was a crisp one-liner. Plimpton's subtle humor worked better with real people than it does with his invented characters.

THE NOVEL's Sidd Finch is an Englishman who lives for a while in a Tibetan lamasery. One of his duties is scaring off the snow leopard that raids the head lama's yak pens. Finch does so with a stone that he hurls at incredible velocity and with precise control. But:

"When you get very good at something you wonder how you can apply it to something more gainful and useful . . . baseball came to mind."

Lugging his French horn (which he plays as well as the late, great Dennis Brain) Finch goes to Maine where he demonstrates "the art of the pitch" to a New York Mets scout. Invited to spring training, Finch throws the ball so fast that his catcher can't see it and simply hopes the ball hits his mitt. It does. Every time. Finch, everyone agrees, would be unhittable. He would "change the face of the future."

But will Sidd go pro? His reluctance is strung out over 10 chapters during which his character is fleshed out. His passion for Eastern mysticism is exhaustively detailed. He moves in with the novel's narrator who has writer's block, something the prolific Plimpton surely has not experienced.

Finch has a decorous romance with a sexy lady golfer. He agrees to play, but in batting practice he loses his perfect control and throws the ball right through the batting cage. Fearful that he'll kill a batter with a wild pitch, Finch takes off, enters a monastery, is talked into giving it another try by his girlfriend, returns to Shea Stadium, and fans 27 St. Louis Cardinals with 81 pitches -- THE perfect game.

The world of baseball is in shock. Opinion is divided on whether Finch is good or bad for the game. A Cardinal fan, who is also a Mafia hit man, decides to "off" Finch; the amusing chase scene is quite funny.

So. What does Commissioner Ueberroth do? Can he let Finch pitch again? Does he? Will big league scouts descend upon Tibet in search of hard-throwing monks? Is Sidd Finch the face of the future?

Find out for yourself. The Curious Case of Sidd Finch is not the rollicking farce I'd hoped for, but it's worth a reading. ::

Jim Brosnan, a major league pitcher from 1954 to 1963, is the author of "The Long Season."